(packaging) Use the packaging loader for tasks

The packaging repo now uses an explicit loader which handles loading the
various rake tasks in packaging. This commit updates the rubycas-server
Rakefile to use the loader instead of a blind glob of the ext/packaging/tasks
directory. We move this load into the rescue LoadError block because the
packaging repo won't always be there.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>